1. Field of the Invention
The present invention relates to an inkjet recording apparatus including plural recording heads, each of which has plural nozzles arranged at a predetermined pitch in a line, wherein the plural recording heads are arranged in a zigzag fashion such that the recording heads are parallel with a main scanning direction while ends of the recording heads are overlapped with each other, and the nozzles of the respective recording heads are driven at a predetermined timing corresponding to the positions of the respective recording heads in a sub scanning direction such that ink is discharged on print paper fed along the sub scanning direction, thereby forming an arbitrary image including an in-line image. More particularly, the present invention relates to an inkjet recording apparatus constructed such that the quality of an obtained image is not deteriorated at positions corresponding to joint parts of the recording heads (overlap parts of the nozzle arrays) of the above-described inkjet recording apparatus even when assembly error of nozzle pitch occurs at the joint parts.
2. Description of the Related Art
It is economically unreasonable that a head of an inkjet recording apparatus, which discharges ink from nozzles to form an image, is constructed as a unitary structure having a length corresponding to the width of print paper. Consequently, small-sized recording heads are normally modularized, and plural modularized recording heads are assembled to construct a large-sized line head having a desired length.
For example, Patent Reference 1 discloses an inkjet printing apparatus including plural inkjet heads, each of which has plural ink nozzles for discharging ink, wherein the ink nozzles are arranged at a predetermined interval in a line, and the inkjet heads are arranged such that the arranging directions of the ink nozzles coincide with each other in a line. The distance between the adjacent inkjet heads may be greater or less than the nozzle pitch of each inkjet head depending upon the assembly accuracy of the inkjet heads. In this case, however, portions of an image corresponding to joint parts of the inkjet heads are merely thickened or thinned by increasing or decreasing the number of ink drops, and therefore, lines or spots are formed in the obtained image. In the inkjet printing apparatus disclosed in Patent Reference 1, the number of ink drops are not merely increased or decreased depending upon the nozzle pitch at the joint parts of the inkjet heads, but the number of ink drops is decided such that ink can be appropriately spread around the joint parts with a statistically desired concentration by a fuzzy control, without fixing the number of ink drops. As a result, a regular shape does not appear at positions of the image formed on print paper, which correspond to the joint parts of the inkjet heads, and therefore, lines or spots are not visible.
[Patent Reference 1] Japanese Unexamined Patent Publication No. 2005-7582
In the case that a large-sized inkjet recording apparatus is constructed by assembling plural small-sized recording heads, which are modularized, in parallel with each other, when the nozzle pitch of the respective recording heads, each of which is constructed as a module, is small, the pitch between the nozzles at the coupling parts of the adjacent recording heads may not be physically maintained. In this case, it is impossible to combine the plural recording heads such that the recording heads are arranged in a line; however, it is possible to two-dimensionally arrange the recording heads in a zigzag fashion such that the nozzle arrays of the respective recording heads are parallel with a main scanning direction, whereby the nozzle arrays at the corresponding ends of the adjacent recording heads are overlapped with each other.
FIG. 1 is a view schematically illustrating the structure of the above-described inkjet recording apparatus. Plural small-sized recording heads 1, which are modularized, are prepared, and the modularized recording heads 1 are arranged side by side in a zigzag fashion along a main scanning direction (in the right-and-left direction in the drawing). In the illustrated example, the inkjet recording apparatus is a single color (for example, black color) inkjet recording apparatus. Approximately 10 of nozzles are overlapped at the joint part 2 (the overlap part of the nozzle arrays) between the adjacent recording heads 1 (modules). Approximately five of the nozzles, among approximately ten of the nozzles, which are overlapped at the joint part 2, are continuous with the nozzles, which are not overlapped, of one of the adjacent recording heads 1, and therefore, approximately five of the front nozzles, which are arranged before the above-mentioned nozzles, are not used. Approximately five of the remaining nozzles, among approximately ten of the nozzles, which are overlapped at the joint part 2, are continuous with the nozzles, which are not overlapped, of the other of the adjacent recording heads 1, and therefore, approximately five of the front nozzles, which are arranged before the above-mentioned nozzles, are not used. Consequently, the distance between the nozzles, which are used, at the ends of the respective recording heads 1 becomes equal to the distance between the nozzles of the recording heads 1.
In the inkjet recording apparatus in which the plural modularized recording heads 1 are arranged in a zigzag fashion along the main scanning direction as described above, print paper 3 is fed in the sub scanning direction (in the upward-and-downward direction in FIG. 1), ink is discharged from the respective nozzles selected from the respective recording heads 1 to the print paper 3 at a predetermined timing corresponding to the positions of the respective recording heads in the sub scanning direction, whereby an in-line image continuous in the main scanning direction is formed on the print paper 3, which will be described below in detail.
First, as shown in FIG. 2A, ink is discharged (printing is performed) from recording heads 1a, which are located at the front part in the sub scanning direction (in the paper feeding direction). Subsequently, as shown in FIG. 2B, ink is discharged from recording heads 1b, which are located at the rear part in the sub scanning direction, at the time when the print paper 3 has been fed by the separation distance of the recording heads 1 in the sub scanning direction from the state shown in FIG. 2A. As a result, as shown in FIG. 2C, an in-line image continuous in the main scanning direction, is formed on the print paper 3.
In the case that the plural recording heads 1 are arranged such that the recording heads 1 are overlapped with each other at the joint parts in the main scanning direction, however, it is necessary that the plural recording heads 1 be arranged in a two-dimensional plane in a zigzag fashion along the main scanning direction as described above. Especially, it is necessary that the respective recording heads 1 be arranged and assembled with high accuracy such that the nozzle pitch between the adjacent recording heads 1 at the joint part 2 is equal to the pitch of the respective recording heads 1.
Furthermore, a step may occur between the recording heads 1 and 1 even in the sub scanning direction depending upon the accuracy of the disposition of the recording heads 1 and the accuracy of the feeding of the paper. The following description will be given in consideration of the fact that misalignment occurs at the joint part 2 in the sub scanning direction.
FIGS. 3A to 3C are enlarged views illustrating images entirely printed on the print paper 3 by the inkjet recording apparatus shown in FIG. 1. In the drawings, the circles indicate dots formed on the print paper 3 by ink drops discharged from one nozzle. Dots formed by one of the recording heads are indicated in outline, and dots formed by the other recording head are indicated in light black. In FIGS. 3A to 3C, nine enlarged images are shown by the combination of assembly error states and misalignments in the sub scanning direction (the middle parts have no misalignment, and the upper and lower parts have misalignments in opposite directions).
When the assembly error in the main scanning direction at the joint part 2 of the above-mentioned adjacent recording heads 1 is 0, as shown in FIG. 3A, the deterioration of the quality of the image at the position corresponding to the middle joint part 2 does not occur even though the misalignment occurs in the sub scanning direction. However, when the distance between the nozzles in the main scanning direction at the joint part 2 of the adjacent recording heads 1 is decreased, as shown in FIG. 3B, the concentration of the overlapped portions of the dots becomes thick, and therefore, a black straight line is formed in the sub scanning direction. Also, when the distance between the nozzles in the main scanning direction at the joint part 2 of the adjacent recording heads 1 is increased, as shown in FIG. 3C, the concentration of the separated portions of the dots does not appear, and therefore, a white straight line is formed in the sub scanning direction. These are important factors that are visibly prominent, and therefore, deteriorate the quality of the image, which are necessary to be improved.
A method of adjusting the assembly error to “0” may be embodied by providing a structure in which the positions of the respective recording heads 1 constituting the line head can be adjusted, and performing the adjustment during the assembly process. However, it is necessary that the plural recording heads 1 be arranged in a zigzag fashion, in two dimensions, and with high accuracy, as described above. For this reason, the adjustment costs are greatly increased.
Consequently, the adjustment is actually carried out with appropriate accuracy.